11.1 Introduction

The purpose of this lab exercise is to familiarize you (in a very basic way) with the analysis of fossil communities. You will have the opportunity to apply some of the ideas which have been discussed in lecture, as well as skills that you have picked up in previous labs. But first, you will have to be introduced to some concepts and terminology.

Note that in the previous paragraph I stated that you will analyze fossil communities, but the lab is entitled "Fossil Assemblages". The reason for the different terms is based on the problem of ecological community description in general. Ecological communities are usually defined and examined on the basis of a small fraction of the organisms that are actually present and interacting, and given the overall poor preservation of organisms, fossil communities are even more difficult to pin down. Therefore a more proper term is "assemblage", because it refers only to the fossilized portion of the original community. Despite the dicey and poor preservation of ecological communities though, we are able to reconstruct in a very general way pictures of past communities. And things become very interesting when we apply that unique paleontological parameter, time. The essential questions are:

• Have ecological communities (as represented by fossil assemblages) changed over Phanerozoic time?
• If yes (and this is not guaranteed), how have they changed? Are there any general patterns?

11.1.1 Definitions

You will need to understand the following terms and concepts as they apply to organisms.

• Planktonic: floating freely in the aquatic environment, and unable to accomplish large-scale movement under own power. These organisms typically have very small Reynolds numbers (Re) (<100). Examples are microcrustaceans (copepods), phytoplankton (diatoms), many larval animals and jellyfish.
• Nektonic: free-swimming animals, capable of large-scale movement in their environments. These animals are of high Re (1,000 to >1,000,000), and include fish, cetacea, squid, etc.
• Benthic (benthonic): living on or in the sediment. Re is of less significance here to many animals, and the diversity can be quite high. Within the benthic environment, animals can be classified as follows:
  • Epifaunal: living on the surface of the sediment, or at the sediment-water interface. Example, oysters.
  • Infaunal: living within the sediment itself. Example, most clams.
  • Epibiotic: living on the body of another animal. Epibiotic organims typically favor a hard substrate to settle on, and sometimes this is the external skeleton of another animal. Examples are encrusting algae and barnacles.
• Deposit feeder: feeding on the sediment itself. Many marine sediments are rich with organics from the water column above, as well as all the benthic biological activity. Animals can make their living off these sediments. Example, marine annelids (an equivalent terrestrial annelid would be the earthworm).
• Suspension feeder: feeding on particles suspended in the water column. Filter feeding is a type of suspension feeding. Some suspension feedesr are passive (example, archaeocyathans), while some are more active (examples, crinoids [sea lilies] and most clams).
• Predator: Actively capturing, subduing, and devouring another animal. No need for examples here.